Projection display devices often include optical elements and a uniform light source to illuminate the optical elements. Many light sources, however, are not sufficiently spatially uniform to illuminate the projection display devices. Light pipes are commonly used to improve the uniformity of the light produced by such non-uniform light sources, thereby creating a uniform light source for illumination optics in projection display devices. Light pipes are generally configured in one of two common forms: (1) as a hollow tunnel, in which a pipe has a highly reflective inner wall (e.g., has a highly reflective coating on its inner wall), or (2) as a solid member, in which a solid glass rod has an optically transparent medium. In form (2), the light pipe relies on total internal reflection (TIR) to contain the light within the solid member. The light pipe may also be (3) a clad light pipe. The clad light pipe is a light pipe that has a thin coating or layer of material (e.g., glass or plastic) that surrounds (except for the ends) the light pipe. The coating or layer has a lower index of refraction as compared to the light pipe.
The light pipe may have an input end (or input face) configured to receive the light, which may be from the light source providing non-uniform light, and an output end (or output face) configured to emit the light. The input and output ends may have an anti-reflective coating to improve the transmission efficiency of the light pipe. As the light passes from the input end to the output end, the light pipe may be configured to allow the light to interfere or mix through multiple reflections. Consequently, the light exiting the output end of the light pipe may be substantially more spatially uniform than the light entering the input end of the light pipe. Accordingly, the light pipe may substantially improve the uniformity of the light provided by the light source, resulting in a highly uniform light source. In projection display devices, the output end of the light pipe is generally imaged to a microdisplay device. The microdisplay device is then re-imaged by a projection lens onto a screen viewed by an audience.
Some drawbacks of using the solid light pipe are that the output face may obtain structural defects (e.g., scratches, edge chips or pits), coating defects (e.g., discoloration) or surface contaminants (e.g., dust, oil, dirt, fingerprints, etc.), all of which alter the image shown on the screen. That is, the edge chips may cause light leakage, “crow's feet” artifacts, image artifacts and bonding problems. Also, the dust may cause dark areas to appear on the screen. For example, the dust may collect on and/or fuse to the output face due to the high temperatures at the input and output faces of the light pipe. The dust may create dark areas on the output face of the light pipe, ultimately resulting in dark areas appearing on the screen, thus adversely affecting the quality of the image viewed by the audience. In the past, the dark areas have been minimized by creating a dust free environment for the input and output faces of the light pipe. This solution, however, is typically inconvenient and may add significant cost and complexity to the apparatus surrounding the light pipe, the optical elements and the entire projection display device.
Another drawback of using a conventional light pipe approach is that the illumination is performed obliquely when using a microdisplay device such as a digital micromirror device (DMD) (e.g., a DMD from Texas Instruments as found in digital light processing (DLP) projectors). In such systems, the DMD plane is tilted with respect to the incoming illumination light and the optical axis of the illumination system. Effectively, this means that the image of the output face of the light pipe is tilted with respect to the DMD plane, and the two planes share only a single line of common focus. In an ideal situation, the two planes would be coincident. Undesirable effects due to this tilted illumination system and non-coincident focus include blurred edges to the lightbox, degraded illumination uniformity and efficiency losses.
Accordingly, it should be appreciated that there is a need for a system and method for providing a uniform source of light. The invention fulfills this need as well as others.